1. Field of the Invention
The present invention relates to an improved polyimide and polyimide film prepared therefrom as well as a manufacturing method of the polyimide film.
2. Description of the Prior Art
Polyimide is known for its outstanding heat resistance, low-temperature resistance, resistance to chemicals, electric insulation property and mechanical strength etc, and has a wide scope of use like, among others, electric insulation materials, heat insulation materials and base films for flexible printed circuit boards (PCB). When used as material for flexible PCB, which is an important use for a polyimide film, the manufactured PCB is often used for a prolonged period in a deformed state or subjected to repeated contraction or bending. Hence, in this field of use, mechanical properties of a polyimide film are very important, especially its tensile strength and anti-bending property are critical.
It is also often the case that polyimide film is used, as in the manufacture of flexible PCB, in a form laminated with a copper foil, adhesive layer and the like. In such a case the problem was that a laminated film as such with its thickness increased had its physical properties, especially repeated flexural strength, markedly deteriorated or lowered.
Hence, it was strongly desired to have the mechanical strength of polyimide film as a base, flexural failure strength in particular, much more improved when it was to be used by being laminated with a copper foil and an adhesive layer.
As will be described later, the present invention consists in mixing some organic phosphorus compound to polyimide, but as far as the present inventors investigated prior arts involving the use of phosphorus compound as an additive to heat resistant synthetic high polymer, they found out Japanese Laid-open Application No. 131249/'82 and Japanese Laid-open Application No. 221454/'85. The former relates to a thermoplastic polyamideimide resin composition in which organic phosphonite is mixed with aromatic polyamideimide and the latter discloses a technique relating to thermoplastic polyetherimide incorporating sterically hindered phenol to polyetherimide, aryl phosphonite, organic phosphonite etc. Polyamideimide and polyetherimide are both thermoplastic resins and, moreover, both techniques are aimed at preventing thermal decomposition of melted thermoplastics. Thus, any of the above-identified prior arts neither discloses nor suggests anyway the technical concept of the present invention.